1. Field of the Invention
The present invention relates to a honeycomb structure and a method of producing the same. More particularly, the present invention relates to a honeycomb structure including a coating layer disposed to cover an outer wall forming a cell structure, and a method of producing the same.
2. Description of Related Art
A ceramic honeycomb structure has been used as a filter for trapping particulate (particularly diesel particulate) contained in automotive exhaust gas (diesel particulate filter: DPF), or a catalyst carrier for supporting a catalyst for purifying nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbons (HC), and the like contained in exhaust gas.
The above honeycomb structure includes a cell structure having a porous partition wall partitioning a plurality of cells serving as fluid channels and an outer wall formed on the outer circumference of the partition wall. The honeycomb structure is produced by extruding a forming clay, drying the extruded product, and firing the dried product, for example. The honeycomb structure (cell structure) inevitably undergoes dimensional errors and deformation in each step. Specifically, since the extruded product is very soft due to the presence of water, a binder, and the like, the extruded product is easily deformed due to its weight. In the case of producing a large product with a diameter of 300 mm, when the product is extruded in the direction perpendicular to the gravitational direction, the outer circumferential cells are crushed due to the weight of the product. On the other hand, when the product is extruded in the direction parallel to the gravitational direction, the lower portion of the formed product is buckled due to the weight of the product. Moreover, since the product may not be uniformly dried or fired in the drying or firing step, deformation may occur due to the difference in shrinkage rate.
It is difficult to hold a honeycomb structure with poor outer circumferential dimensional accuracy at a constant contact pressure when providing the honeycomb structure in a converter casing for providing a DPF or a large carrier. Therefore, technology has been proposed in which a coating layer is disposed to cover an outer wall forming a cell structure to obtain a honeycomb structure with high dimensional accuracy (see patent document 1, for example).
In recent years, it has become necessary to deal with exhaust gas regulations tightened year by year. In view of a demand for reducing fuel consumption while increasing output, a reduction in pressure loss caused by a catalyst carrier or a filter and an increase in exhaust gas purification efficiency have been demanded. In order to deal with such a demand, it is necessary to reduce pressure loss by reducing the thickness of the cell partition wall and to improve purification performance by promptly activating a catalyst after starting an engine. A honeycomb structure having such structural features exhibits low mechanical strength since the partition wall is thin and has a high porosity. In order to prevent deformation, breakage, and the like during use by improving mechanical strength, a method has been proposed in which a reinforcement layer (coating layer) or a shell layer formed of a specific reinforcement material or the like is disposed on the outer circumference of a honeycomb structure (ceramic honeycomb main body) to improve the mechanical strength of the honeycomb structure (see patent document 2, for example).                [Patent document 1] JP-UM-A-63-144836        [Patent document 2] Japanese Patent No. 2613729        